The present invention relates to a power assisted steering system and is particularly concerned with such a system of the hydraulic fluid actuated type having an open centre power assistance control valve which is adjustable in response to a steering input torque to control fluid flow to a servo motor in a manner which provides power assistance appropriate to the steering input. It is generally recognised that in such type steering systems, pressurised hydraulic fluid in passing through ports, recesses and over control edges in the open centre power assistance valve can generate considerable noise which, although not detrimental to the operation of the valve, is considered a nuisance; furthermore, the resistance to fluid flow through the open centre valve tends to develop a back pressure in the system which is technically undesirable, particularly due to heat which can be generated in the system. These undesirable features of noise and back pressure/heat generation are of special concern when the power assistance control valve is in its open centre condition as it will be for the major part of the period for which the system is in use and which condition is consistent with there being no change in steering (that is a steering manoeuvre is not being effected). It is an object of the present invention to provide an hydraulic fluid actuated power assisted steering system having an open centre power assistance control valve which alleviates the generation of noise and back pressure from said valve, particularly when the valve is in its open centre condition, and which system is of a relatively uncomplicated and economical construction and may lend itself to being achieved by a simple modification to existing hydraulic power assisted systems having open centre power assistance valves.